None of the constants are exported by default; you have to ask for them explicitly.
Some of the constants are grouped into bundles that you can grab all at once; alternatively you can just take the individual constants,
one by one.

If you wish to import a group,
your use statement should look something like this:

use Crypt::OpenPGP::Constants qw( :group );

Here are the groups:

packet

All of the PGP_PKT_* constants. These are constants that define packet types.

Other exportable constants, not belonging to a group, are:

DEFAULT_CIPHER

DEFAULT_DIGEST

DEFAULT_COMPRESS

Default cipher, digest, and compression algorithms, to be used if no specific cipher, digest, or compression algorithm is otherwise specified.